In the present world, cancer is a disease showing the highest incidence and mortality rate. It seriously threatens the health of human beings and their life, and remains a puzzle for researchers. In the early 1970s, scientists knew nothing about the cause of formation of cancers. They had to find anticancer remedies blindly. In 1966, National Cancer Institute began to fund the chemical drug screening project. Thousands upon thousands of chemicals which might have the potential of treating cancer were tested one by one. As a result, many chemotherapy drugs were developed, such as, amino methyl folic acid, cyclophosphamide, cisplatin, fluorouracil, pacilitaxel etc. Though this kind of cytotoxic drugs can ameliorate or eliminate some pathological symptoms of the disease, continuous treatment usually causes serious multi-rug resistance, leading to less curative effect on treatment. What make things worse is that the cancer will develop resistance to these kinds of drugs, and then future treatment may be impossible.
In 1971, Dr. Folkman put forward the angiogenesis theory at the first time. He made the following bold hypothesis: 1) Tumor growth depends upon angiogenesis, 2) Tumor can stimulate the formation of this kind of blood vessel initiatively, 3) Tumor can secrete some chemical substance which can lure the blood vessel to grow towards the tumor, and to grow the branches. The growth of the solid tumor depends on the quantity of neoplastic cells and endothelial cells of tumour blood vessel. These two kinds of cells depend on each other for mutual existence. The fluctuation of the quantity of any kind of cells can inevitably cause the corresponding fluctuation of another one. Hence, any drug which can inhibit either neoplastic cells or endothelial cells of tumour blood vessel is useful in the treatment of cancers. Inhibiting neoplastic cells is chemotherapy using mostly cytotoxic drugs, and inhibiting endothelial cells is anti-angiogenic therapy which is the most noticeable one lately. According to the hypothesis of Dr. Folkman, the growth and transfer of solid tumor relies on the neovascularization. So, if the formation of tumor blood vessel is inhibited, then tumor cells will die because of lacking of blood and oxygen supply; and sequentially, the tumor's growth is retarded and the transfer of the tumor is inhibited. Now it was proved that angiogenesis of new blood vessels, is a requirement for a tumor to grow. A tumor having a volume of less than 1-2 mm3 can survive through obtaining nutrition from the surrounding tissues by osmosis. The tumor grows slowly at that time because its further development must rely on angiogenesis to obtain sufficient nutrition. Tumor angiogenesis is a dynamic multi-step process, which involves retraction of pericytes from the abluminal surface of the capillary, release of proteases from the activated endothelial cells, degradation of the extracellular matrix (ECM) surrounding the pre-existing vessels, endothelial cell migration toward an angiogenic stimulus and their proliferation, formation of tube-like structures, fusion of the formed vessels and initiation of blood flow. This process is regulated by both the internal secretion factor of basement nerve and growth factors expressed by tumor cells and tumor matrix cells.
Anti-angiogenic therapy attacks similar vascular endothelial cells that surround different tumors. Except for the higher proliferation rate of tumor vascular endothelial cells, there is no apparent difference between them and normal ones. Normal vascular endothelial cells have longer lifetime and more stable genotype. Except for the nerve cells, endothelial cell is one kind of the cells that have the longest lifetime. Among the endothelial cells in the adulthood vascular wall, only around 0.01% of them are in the state of division. Tumor vascular endothelial cell differs from normal vascular endothelial cell in several ways. Tumor vascular endothelial cell has a proliferation rate that is on average 50-fold higher than normal vascular endothelial cell and is therefore less mature. Consequently, vascular inhibitory factors have relative specificity for tumor blood vessels, while no noticeable effect on the vascular of normal tissues. Compared with traditional chemotherapy, which directly targets tumor cells, anti-angiogenic therapy has some significant advantages: 1) Anti-angiogenic drugs have good specificity because angiogenesis is initiated once tumor occurs. Anti-angiogenic drugs directly targets neovascular endothelial cells, therefore, thousands of tumor cells will die due to lack of oxygen as long as any one of vessels that causes occlusion is destroyed. Related studies show that 99% of tumor cells die in ischemic site after two hours treated by anti-angiogenic drugs. 2) Because the vascular endothelial cells are exposed themselves to the blood flow, they can be targeted by the drugs directly. The anti-angiogenic drugs will not kill the tumor cells directly and will just change the formation and growth rate of the cells. The therapeutic dosage of the anti-angiogenic drugs is so small that is only 1/10- 1/100 of maximum tolerated dose (MTD). Because of the high therapeutic effectiveness with small dosage, no adverse effects are aroused as those by radiotherapy and chemotherapy. 3) The endothelial cells have relatively stable gene expressing and are not easy to produce drug resistance. The proliferation rate of the vascular endothelial cells is quicker several tens times than that of the normal tissues. Angiogenesis inhibitors have the selective effects on the tumor vascular endothelial cells, which proliferate speedily, and have very limited effects on the normal tissues. The angiogenesis inhibitors have great advantages.
The combretaceae family of shrubs and trees, found in tropical or subtropical areas, is well represented in traditional medical practices. There are 25 known combretum genera used for treating Hansen's disease and cancers in Africa and India. In the end of 1970s, after widespread screening, National Cancer Institute found that combretum genera plants can strongly inhibit the P388 lymphocytic leukemial cell. From the beginning of 1980s, there was a wide interest in studying this kind of plant. During this period, Dr. G Robert Pettit., the director of Cancer Research Institute of Arizona State University, and his colleagues isolated combretastatins from the African willow tree Combretum caffrum which has been used by the Zulus as herbal remedies and as paint for spears. In the Journal of Canadian Chemistry Dr. George R. Pettit stated that the bark of the tree had anti-tumor activity. Afterward, not only many compounds having high activity are isolated and identified, but also the research on their pharmacological mechanism and modifications of their structures have been developed. The group of Dr. Pettit firstly began an in-depth study in this field. This group studies the combretum genera plants, which resulted in isolation of a series of active phenanthrenes, stilbenes, and bibenzyls. Discovery of the very potent cell growth and tubulin inhibitors combretastatins A-1 and A-4 (hereinafter referring to CA-4 and CA-1, represented by formula II) was especially important. Both proved to be exceptionally strong inhibitors of tubulin polymerization (U.S. Pat. No. 5,561,122; WO 9935150).

There is extraordinary interest in studying the Combretastatins, though it was not so long before it was discovered. It is not only because they have higher anti-tumor activity, but also because they are small natural products which inhibit tubulin polymerization and angiogenesis. The study of the mechanism of action of the CA-4 shows that the A-ring and B-ring bind to α-tubulin and β-tubulin respectively, which cause the collapse of endothelial cells of tumor vascular. CA-4 suppresses tumor growth by attacking new-born blood vessel via inhibiting tubulin polymerization, because the blood vessels supply necessary oxygen and nutrients for tumor growth.
CA-4 can enter the endothelial cells that line the blood vessels of tumor cells. In tumours, these cells are immature and thus particularly sensitive to Combretastatins effects compared to the endothelial cells in normal tissue. Once entering the endothelial cells, Combretastatins destroys the internal skeleton of the cells and changes their shape from flat to round, effectively plugging the capillaries that feed the tumours, resulting in big area of tumor ischemia, sequentially, resulting in tumor regression. According to the experiences of prevenient experiments and clinical trials, no any traditional anticancer drug can enter this site so far. This theory was proven in the CA-4 phase I clinical trial so far. For every individual cancer patient, within 4 to 6 hours after the treatment of CA-4, the blood flow decreased obviously, and more than 95% tumor cells died. In addition to development of CA-4 as a systemic agent for the treatment of cancer, it is also being developed as a topical application for use in the treatment of various ocular diseases, including age-related macular degeneration and proliferative diabetic retinopathy. CA-4 may also have potential for the treatment of psoriasis and arthritis. CA-4 also shows effect in boosting immunity, so it may be also useful in the treatment of diseases related to AIDS (WO02058535; U.S. Pat. No. 6,773,702).
Recently, CA-4 shows exciting property in shutting down tumor vasculature as a tumor vascular targeting agent. (Thorpe P E. Clin Cancer Res. 2004 Jan. 15, 10(2):415-27; West C M, Price P. Anticancer Drugs. 2004 Mar. 15(3):179-87; Young S L, Chaplin D J. Expert Opin Investig Drugs. 2004 Sep. 13(9): 1171-82.) So developing new CA-4 analogs is becoming a highlighted subject. For example, Oxigene Inc. has developed a series of functionalized stilbene derivatives (U.S. Pat. No. 6,919,324).
It is well known that introducing fluorine to a bio-active molecule will change its biological activity, but it is uncertainty whether it will result in increase or decrease of activity.
For example, the fluorocombretastain synthesized by Sigma-Tau Industrie Farmaceutiche Riunite S.P.A, which was introduced one or two fluorine atoms in the double-bond bridge, showed no any difference from CA-4 in activity.
Therefore, finding new Combretastain derivatives with higher activity is an exigent task for us.